1. Field of the Invention
The present invention relates to a Micro Electro Mechanical Systems (MEMS) device formed inside a hermetic chamber having a getter film.
2. Description of the Related Art
Silicon-based MEMS devices are rapidly progressing in a growing number of technical fields. In recent years, MEMS devices are utilized not only in micro turbines and sensors, but also in communications and medical fields. Getter technology represents a fundamental element of MEMS technology. Getters are important for stabilizing a performance of MEMS devices formed inside hermetic chambers. Improved getter technology is indispensable to an advancement of many fields, especially fields requiring miniaturization in applications such as sensors.
Conventional getters are formed by vapor deposition on a bottom of a cavity made in a glass substrate. A hermetic chamber is formed, for example, by bonding silicon and glass substrates by anodic bonding. The getter absorbs an amount of oxygen or other residual gases remaining after the anodic bonding, as described in Japanese Published Unexamined Patent Application No. 10-122869.
Japanese Published Unexamined Patent Application No. 10-213441 discusses a method of reactivating a getter formed on a bottom of a cavity made in a glass substrate, wherein the reactivation is performed after sealing the chamber by applying difference in electrical potentials to the getter to produce an electric current therein.
Unfortunately, a location of the getter film inside the hermetic chamber may affect various aspects of performance of the MEMS device. For example, in the case where a portion of a getter is too close to an electrode formed on a MEMS device, even though no physical contact is made therebetween, electric noise may transmit from the electrode via the getter film coupled capacitively to an electrode of a sensor or a driver, thereby impairing the operation of the MEMS device.
Such problems are especially pronounced in the case where a miniature MEMS device is utilized in, for example, a resonator gyroscope chip having a substantially square cross-section occupying 100 mm2 or less when viewed from the front, as illustrated in FIG. 1. In particular, the miniaturization of a device requires a smaller volume of hermetic chamber. Gases such as residual gases and gases released into the hermetic chamber during production remain inside the hermetic chamber. The smaller hermetic chamber volume results in stronger influences of these gases on the pressure inside the hermetic chamber. As a result, a detection sensitivity of a MEMS sensor, for example, may be noticeably impaired. Thus, the technical significance of the getter region for miniature MEMS devices is particularly high. Unfortunately, it is very difficult to form conventional getters that solve the problems recited above while maintaining the required gas absorption capabilities.